File size dilemma in Photoshop for print-quality large banner

Question

I'm an intern at a marketing department and got an assignment to update a banner with new information, and was directed to a pdf file to edit. It was only 3MB, which seemed very low for a large print-quality file (about 100 x 30 inches in size), but I opened it in Photoshop anyway (the original PSD and InDesign files are apparently misplaced).

I edit the image, flatten it back down without changing any settings (300 dpi, CMYK, the works), and suddenly I can't save a new file as a pdf since it's too large, almost 3 gigabytes. With no layers and exactly the same settings as the original pdf, I have no idea how to compress the file back down without losing quality. How was it only 3MB in the first place? I have no idea, and neither does anyone else, it seems.

Is there any way I can reduce the size without compromising the print quality? The lowest I can get to is 50MB after changing dpi to 150, but I'm not sure it's safe to tamper with that either.

Answer 1

The original file wasn't pixel-based but vector.

Pdfs can contain all kinds of data, not just pixel images. When you open a .pdf in Photoshop, you force the program to rasterise it into a pixel image. Figures, that is the only kind of image Photoshop can process and edit.

Actually, .pdfs aren't meant to be edited. It is in their very core to be immutable, sacrificing editability for a Portable Document Format. Do note that some programs (Illustrator springs to mind) create .pdfs that retain their editability.

The only way to reproduce a similarly sized .pdf is to create one in a vector or mixed application like Illustrator or InDesign, using as little pixel imagery as possible in favour of vectors.

Answer 2

The selected answer is of course the best one - however for those who don't have access to illustrator or something else which can edit a pdf in it's vector format, a few suggestions for when you have to go to a rasterized format and want to reduce filesize:

1. If you have to save as PDF, check the image encoding settings in the save dialog. There are usually JPEG, PNG and other image format options within PDF. Go for JPEG for most cases, and for a large file, try 75% quality. If you have to go lossless, stick with PNG.

2. If you can, save as an alternative file format like PNG or JPEG. Photoshop and the like are very good at creating lower filesizes without losing significant visual quality for JPEG. There are also tweakable options for saving out PNG's. These may not be possible within the PDF container format.

3. Reduce colour depth, if possible. Depending on your printer, you may not experience any difference between 24-bit and higher bitdepths like 48-bit.

4. Remove transparency (alpha channel), if possible. This is, in most colour formats, 1/4 of your image filesize, so saving out the file without transparency (default in JPEG, optional in PNG/TIFF/etc) will reduce filesize.

5. If all else fails, consider lowering your DPI or raw pixel size. Consider what is noticable for your output device, be it printer or screen, and adjust accordingly.